U.S. patent application number 13/495559 was filed with the patent office on 2013-12-19 for led strobes with fixed pulse width.
The applicant listed for this patent is Kenneth E. Savage, JR.. Invention is credited to Kenneth E. Savage, JR..
Application Number | 20130335229 13/495559 |
Document ID | / |
Family ID | 48672841 |
Filed Date | 2013-12-19 |
United States Patent
Application |
20130335229 |
Kind Code |
A1 |
Savage, JR.; Kenneth E. |
December 19, 2013 |
LED STROBES WITH FIXED PULSE WIDTH
Abstract
An LED strobe notification device and method for operating an
LED strobe notification device is provided. The LED strobe
notification device is configured to generate an output at two or
more candela settings and is configured to generate the output at
the two or more candela settings having a human-perceived pulse
duration with a fixed width. The LED strobe device includes: an LED
strobe element; and an LED controller in communication with the LED
strobe element with the LED controller configured to: receive a
candela selection for the LED strobe element, the candela selection
selected from the two or more candela settings; receive a command
to activate the LED strobe element of the LED strobe notification
device; and in response to receiving the command, control the LED
strobe element in order to generate the output from the LED strobe
element having the human-perceived pulse duration with the fixed
width.
Inventors: |
Savage, JR.; Kenneth E.;
(Fitchburg, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Savage, JR.; Kenneth E. |
Fitchburg |
MA |
US |
|
|
Family ID: |
48672841 |
Appl. No.: |
13/495559 |
Filed: |
June 13, 2012 |
Current U.S.
Class: |
340/577 ;
340/815.45 |
Current CPC
Class: |
H05B 45/37 20200101;
G08B 17/00 20130101; G08B 17/06 20130101; Y02B 20/30 20130101; H05B
45/00 20200101; G08B 5/38 20130101; Y02B 20/346 20130101 |
Class at
Publication: |
340/577 ;
340/815.45 |
International
Class: |
G08B 17/12 20060101
G08B017/12; G09F 9/33 20060101 G09F009/33 |
Claims
1. An LED strobe notification device configured to generate an
output at two or more candela settings, the output at the two or
more candela settings having a human-perceived pulse duration with
a fixed width, the LED strobe notification device comprising: an
LED strobe element; and an LED controller in communication with the
LED strobe element and configured to: receive a candela selection
for the LED strobe element, the candela selection selected from the
two or more candela settings; receive a command to activate the LED
strobe element of the LED strobe notification device; and in
response to receiving the command, control the LED strobe element
in order to generate the output from the LED strobe element having
the human-perceived pulse duration with the fixed width.
2. The LED strobe notification device of claim 1, further
comprising a candela selector configured to receive a manual
indication of the candela selection.
3. The LED strobe notification device of claim 1, further
comprising a communication interface configured to receive a
candela selection command from an alarm control panel, the candela
selection command indicative of the candela selection.
4. The LED strobe notification device of claim 1, wherein the LED
controller is configured to generate the output having the
human-perceived pulse duration with the fixed width at the two or
more candela settings by varying an intensity of the output of the
two or more candela settings.
5. The LED strobe notification device of claim 4, wherein the LED
controller is configured to vary the intensity of the output of the
two or more candela settings by modifying drive current to the LED
strobe element.
6. The LED strobe notification device of claim 1, wherein the LED
controller is configured to generate the output having the
human-perceived pulse duration with the fixed width at the two or
more candela settings by pulsing the output of the LED strobe
element at a rate faster than humanly perceptible.
7. The LED strobe notification device of claim 6, wherein the LED
controller is configured to pulse the output by turning the LED
strobe element on and off at the rate faster than humanly
perceptible.
8. The LED strobe notification device of claim 1, further
comprising a pulse width selector configured to receive a manual
indication of the fixed width.
9. The LED strobe notification device of claim 1, further
comprising a communication interface configured to receive a pulse
width selection command from an alarm control panel, the pulse
width selection command indicative of the fixed width.
10. A method for operating an LED strobe notification device, the
LED strobe notification device comprising an LED strobe element and
an LED controller, the LED strobe element configured to generate an
output at two or more candela settings, the output at the two or
more candela settings having a human-perceived pulse duration with
a fixed width, the method comprising: receiving a candela selection
for the LED strobe element, the candela selection selected from the
two or more candela settings; receiving a command to activate the
LED strobe element of the LED strobe notification device; and in
response to receiving the command, controlling the LED strobe
element in order to generate the output from the LED strobe element
having the human-perceived pulse duration with the fixed width.
11. The method device of claim 10, wherein receiving a candela
selection for the LED strobe element comprises receiving a manual
indication of the candela selection via a candela selector on the
LED strobe notification device.
12. The method device of claim 10, wherein receiving a candela
selection for the LED strobe element comprises receiving a candela
selection command from an alarm control panel, the candela
selection command indicative of the candela selection.
13. The method device of claim 10, wherein controlling the LED
strobe element in order to generate the output from the LED strobe
element having the human-perceived pulse duration with the fixed
width comprises varying an intensity of the output of the two or
more candela settings.
14. The method device of claim 13, wherein varying the intensity of
the output of the two or more candela settings comprises modifying
drive current to the LED strobe element.
15. The method device of claim 10, wherein controlling the LED
strobe element in order to generate the output from the LED strobe
element having the human-perceived pulse duration with the fixed
width comprises pulsing the output of the LED strobe element at a
rate faster than humanly perceptible.
16. The method device of claim 15, wherein pulsing the output of
the LED strobe element comprises turning the LED strobe element on
and off at the rate faster than humanly perceptible.
17. The method device of claim 10, further comprising receiving a
manual indication of the fixed width.
18. The method device of claim 10, further comprising receiving a
pulse width selection command from an alarm control panel, the
pulse width selection command indicative of the fixed width.
19. A fire alarm system comprising: a fire alarm control panel
comprising a system controller configured to send an activation
command; and an LED strobe notification device configured to
generate an output at two or more candela settings, the output at
the two or more candela settings having a human-perceived pulse
duration with a fixed width, the LED strobe notification device
comprising: a communication interface configured to receive the
activation command; an LED strobe element; and an LED controller in
communication with the LED strobe element and configured to:
receive a candela selection for the LED strobe element, the candela
selection selected from the two or more candela settings; and in
response to receiving the activation command, control the LED
strobe element in order to generate the output from the LED strobe
element having the human-perceived pulse duration with the fixed
width.
20. The fire alarm system of claim 19, wherein the communication
interface is further configured to receive a candela selection
command from the fire alarm control panel, the candela selection
command indicative of the candela selection.
Description
TECHNICAL FIELD
[0001] The present description relates to strobe notification
devices. This description more specifically relates to an LED
strobe notification device configured to generate an output at two
or more candela settings where the pulse duration at the two or
more candela settings is a fixed width.
BACKGROUND
[0002] Fire alarm devices such as audible horns (audible/visible or
A/V), loudspeakers (speaker/visible or S/V) and visible strobes
(visible only or V/O), are referred to as "notification
appliances." Typically, a fire alarm control panel (FACP) drives
these devices over one or more "notification appliance circuits"
(NACs). The strobes are used, for example, as an alert for the
hearing-impaired, or for those in a high noise environment.
[0003] One type of strobe uses a flash tube (also called a flash
lamp). Typically, the flash tube is an electric glow discharge lamp
designed to produce extremely intense, incoherent, full-spectrum
white light for very short durations. Flash tubes are made of a
length of glass tubing with electrodes at either end and are filled
with a gas that, when triggered, ionizes and conducts a high
voltage pulse to produce the light. Xenon is an example of the gas
that can fill the flash tube, with a Xenon flash tube producing a
high-intensity light (such as hundreds of thousands of lumens) for
a very short duration pulse (such as hundreds of microseconds).
Xenon flash tubes use a high voltage storage element, such as an
electrolytic capacitor, that can be charged several hundred volts
to provide energy for the flash. Xenon flash tubes also use a
trigger voltage that is in the several thousand volt range to start
the gas discharge.
[0004] Another type of strobe is Light Emitting Diode (LED)-based.
An LED-based strobe cannot generate light at as high of an
intensity as a Xenon-based strobe. Instead, LED-based strobes
generate a lower intensity light (such as hundreds to thousands of
lumens) for a longer period of time (such as tens to hundreds of
milliseconds). In this way, the LED-based strobes can generate a
comparable amount of light energy, as measured in candela, as a
Xenon-based strobe. Further, an LED-based strobe is a semiconductor
device that can be run off a lower voltage than a Xenon-based
strobe, thus eliminating the high voltage circuitry. A capacitor
may still be used for energy storage in the LED-based strobe,
albeit for a lower output voltage. Because of its physical
characteristics, an LED-based strobe can be turned on either
continuously or pulsed. Finally, in contrast to flash-tube based
strobes, LED-based strobes typically have a longer usable lifetime.
However, LED-based strobes still lack the extremely intense light
output generated by a flash tube based strobe.
SUMMARY
[0005] In one aspect, an LED strobe notification device configured
to generate an output at two or more candela settings is provided.
The LED strobe notification device is configured to generate the
output at the two or more candela settings having a human-perceived
pulse duration with a fixed width. The LED strobe notification
device comprises: an LED strobe element; and an LED controller in
communication with the LED strobe element. The LED controller is
configured to: receive a candela selection for the LED strobe
element, the candela selection selected from the two or more
candela settings; receive a command to activate the LED strobe
element of the LED strobe notification device; and in response to
receiving the command, control the LED strobe element in order to
generate the output from the LED strobe element having the
human-perceived pulse duration with the fixed width. The candela
selection may be input via a candela selector, which may be located
on the LED strobe notification device, or may be input via a
communication interface, which is configured to receive a candela
selection command from an alarm control panel, with the candela
selection command indicative of the candela selection. The LED
controller is configured to generate the output by varying an
intensity of the output of the two or more candela settings (such
as by modifying drive current to the LED strobe element).
Alternatively, the LED controller is configured to generate the
output by pulsing the output of the LED strobe element at a rate
faster than humanly perceptible (such as by turning the LED strobe
element on and off at the rate faster than humanly
perceptible).
[0006] In another aspect, a method for operating an LED strobe
notification device is provided. The method operates the LED strobe
notification device that comprises an LED strobe element and an LED
controller, with the LED strobe element configured to generate an
output at two or more candela settings, the output at the two or
more candela settings having a human-perceived pulse duration with
a fixed width. The method comprises: receiving a candela selection
for the LED strobe element, the candela selection selected from the
two or more candela settings; receiving a command to activate the
LED strobe element of the LED strobe notification device; and in
response to receiving the command, controlling the LED strobe
element in order to generate the output from the LED strobe element
having the human-perceived pulse duration with the fixed width.
[0007] In still another aspect, a fire alarm system comprising a
fire alarm control panel and an LED strobe notification device. The
fire alarm system comprises a system controller configured to send
an activation command. The LED strobe notification device is
configured to generate an output at two or more candela settings,
with the output at the two or more candela settings having a
human-perceived pulse duration with a fixed width. The LED strobe
notification device comprises: a communication interface configured
to receive the activation command; an LED strobe element; and an
LED controller in communication with the LED strobe element. The
LED controller is configured to: receive a candela selection for
the LED strobe element, the candela selection selected from the two
or more candela settings; and in response to receiving the
activation command, control the LED strobe element in order to
generate the output from the LED strobe element having the
human-perceived pulse duration with the fixed width.
[0008] Other systems, methods, features and advantages will be, or
will become, apparent to one with skill in the art upon examination
of the following figures and detailed description. It is intended
that all such additional systems, methods, features and advantages
be included within this description, be within the scope of the
invention, and be protected by the following claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a schematic diagram illustrating a fire alarm
system.
[0010] FIG. 2 is a schematic diagram of the system of FIG. 1,
further illustrating details of a system controller and one example
of a strobe device.
[0011] FIG. 3 is a schematic diagram of the system of FIG. 1,
further illustrating details of a system controller and another
example of a strobe device.
[0012] FIG. 4 illustrates an expanded block diagram of the strobe
device (including strobe element and associated circuitry)
illustrated in FIG. 2.
[0013] FIG. 5 illustrates an expanded block diagram of the strobe
device (including strobe element and associated circuitry)
illustrated in FIG. 3.
[0014] FIG. 6a-b is one example of a pulse duration with a fixed
width at two or more candela settings.
[0015] FIG. 7a-b is another example of a pulse duration with a
fixed width at two or more candela settings.
[0016] FIG. 8 is an example of a block diagram of the energy
storage, LED, and flash control circuitry.
[0017] FIG. 9 is another example of a block diagram of the energy
storage, LED, and flash control circuitry.
DETAILED DESCRIPTION
[0018] A system embodying one example of the present invention is
illustrated in FIG. 1. The system includes one or more notification
appliance circuits (NACs), i.e., networks 16, having alarm
condition detectors D and alarm system notification device A.
Alternatively, the detectors and notification devices may be on
separate networks. A system controller (such as a fire alarm
control panel (FACP)) 14 may monitor the detectors D.
[0019] The system controller 14 may monitor the alarm condition
detectors D. When an alarm condition is sensed, the system
controller 14 may signal the alarm to the appropriate notification
appliances A through the one or more appliance circuits.
Notification devices may include, for example, a visual alarm (such
as a strobe), an audible alarm (such as a horn), or a combination
thereof.
[0020] Although not necessary for carrying out the invention, as
shown, all of the notification devices in a network are coupled
across a pair of power lines 18 and 20 that advantageously also
carry communications between the system controller 14 and the
detectors D and notification devices A.
[0021] The system controller 14 may comprise a fire alarm control
panel and may use one or more commands to signal the alarm to the
appropriate notification appliances A. Examples of commands issued
for a system with addressable notification appliances are disclosed
in U.S. Pat. No. 6,426,697, which is hereby incorporated by
reference in its entirety. Alternatively, the communication line to
the device may be separate from the power line. In still an
alternative embodiment, the system may include non-addressable
notification appliances. The communications channel may comprise,
for example, a wireless link, a wired link or a fiber optic
link.
[0022] Further, the system controller 14 may send one or more
commands relating to diagnostics, status, or other non-alarm type
events. For example the system controller 14 may send a command
related to the identification, the configuration, and/or the status
of the notification appliances A. Moreover, the notification
appliances A may respond in kind.
[0023] One, some, or all of the notification devices A may comprise
a strobe device. The strobe device may be an addressable strobe
notification device (e.g., the strobe notification device has a
uniquely assigned address) or a non-addressable strobe notification
device. As discussed in more detail below, the fire alarm control
panel may send a command to one or more strobe devices to active
the strobe element associated with the strobe device.
[0024] FIG. 2 is a schematic diagram of a part of the system shown
in FIG. 1, further illustrating details of the system controller 14
of the fire alarm control panel and one of the notification
appliances. The system controller 14 includes a processor 36, a
memory 38, a user interface 40, and a device interface 42. The
processor 36 may comprise a microprocessor, a microcontroller, a
digital signal processor, an application specific integrated
circuit (ASIC), a field programmable gate array, a logical digital
circuit, or other now known or later developed logical processing
capability. The processor 36 may work in combination with the
memory 38 in order to monitor part or all of the fire alarm system,
including one or more of the appliance circuits (such as one or
more notification appliance circuits, one or more detector
circuits, and/or one or more notification appliance/detector
circuits). In addition, the memory may include one or more look-up
tables (or other data structures) used for configuration.
[0025] User interface 40 may be used by an operator to control
configuration and/or operation of the alarm condition detectors D
and alarm system notification appliances A. Moreover, device
interface 42 may be an example of a communications interface, and
may comprise the interface between the system controller 14 and the
alarm condition detectors D and alarm system notification
appliances A in the one or more appliance circuits.
[0026] FIG. 2 further depicts strobe device 30 in greater detail.
The strobe device 30 connects to the network 16 via a network
interface (communication connection) 24. The strobe device 30
receives one or more commands from the system controller 14, such
as a candela selection command and/or a pulse width selection
command, as discussed in more detail below. The controller in the
strobe device 30 may comprise an LED flash circuit controller 26,
which processes the one or more commands. Although shown
separately, the memory 32 may be integrated with the LED flash
circuit controller 26. The strobe device 30 further includes LED
strobe element and associated circuitry 44. One example of an
LED-based strobe element is disclosed in U.S. Patent Application
No. 2008/0272911, herein incorporated by reference in its
entirety.
[0027] FIG. 2 further illustrates candela selector 46, which may
comprise an input device on the strobe device 30 through which to
input the selection of the candela output. Candela selector 46 may
comprise one or more switches, such as a set of jumpers or a DIP
switch, thereby allowing a manual setting of the candela output
(such as 15 candela, 30 candela, 110 candela, etc.). In an
alternate embodiment, the candela setting for the strobe device 30
may be set via a command from a fire alarm panel, such as the
system controller 14. In particular, the strobe device 30 may
receive a candela selection command, with the candela selection
command indicative of the candela setting. In still an alternate
embodiment, the manual candela setting may be overwritten upon
command from the system controller 14 (such as by using candela
selection command).
[0028] In an embodiment, an indicator 34, such as a flashing LED,
may indicate the currently configured candela setting, for example,
upon command from the control panel 14, upon a local manual command
such as a pushbutton (not shown), on a periodic basis, always, or
upon some other event.
[0029] LED flash circuit controller 26 is configured to determine
the candela setting (such as via the candela selector 46 or via a
command received from the system controller 14). The LED flash
circuit controller 26 is further configured to control the LED
strobe 44 in order to generate an output at the desired candela
setting and having a human-perceivable pulse duration with a fixed
duration. LED strobe 44 may be separate from indicator 34 (which
may also be an LED). As discussed in more detail below, the LED
flash circuit controller 26 may generate a continuous output for an
entire fixed pulse duration (see FIGS. 6a-b) or may generate a
flickering output for the fixed pulse duration (see FIGS. 7a-b).
The flickering output is at a greater frequency than the human eye
can recognize. So that, in either instance (continuous output or
flickering output), the human eye perceives the output as
continuous for the fixed pulse duration.
[0030] When using strobes as an alarm notification device, there
may be a synchronization requirement that all visible notification
appliances flash within a common predetermined time frame (such as
a 10 millisecond time frame). Xenon flash tube strobes typically
have a flash duration that lasts for hundreds of microseconds. So
that, even when outputting at different candela ratings, the flash
duration of the Xenon flash tube strobes is mostly independent of
the amount of light output by the strobe (candela rating). In
contrast, LED-based strobes have a very different output profile
than a Xenon tube. The power capability of even the newest high
brightness LEDs is limited relative to that of the Xenon tube, with
current ranges in the 4 to 30 W area for single element packages.
For example, in an LED-based strobe element, the light output from
the LED is typically specified with a given luminosity at a rated
continuous current. Further, the relationship of luminosity to
current may be linear around the rated current, and then may become
non-linear as current increases and luminosity falls off as
junction temperatures increase.
[0031] This means that the LED-based strobes cannot flash at the
same intensity found in the peak of the Xenon tube flash, and in
fact are orders of magnitude less bright. Available LEDs can
achieve candela ratings in the range required in mainstream fire
and mass notification applications by lengthening the pulse width,
again by orders of magnitude. However, lengthening the pulse width
creates a qualitatively different type of light output pulse.
Presently, the standards UL1971 and NFPA 72 allow up to a 0.2
second pulse with a maximum 40% duty cycle, 1-2 Hz flash rate. A
LED-based strobe that uses a fixed amount of current and different
pulse widths to achieve different candela may have issues complying
with the 10 millisecond synchronization requirement in UL1971. For
example, the strobe may use a pulse width of 15 milliseconds for a
15 candela strobe setting and 100 milliseconds for 110 candela
setting. If these two strobes were mixed in a given room area,
there is a possibility of flicker between the two strobes since
there is an 85 millisecond difference in flash duration. In this
way, the LED strobes may not be synchronized.
[0032] In one embodiment, the strobe device 30 generates an output
at multiple candela settings with a fixed pulse width in each of
the multiple candela settings. One example of a fixed pulse width
is that the variation in the pulse width between a first LED output
at a first candela setting differs from a second LED output at a
second candela setting is 10 milliseconds or less. In this way, the
fixed pulse width for the multiple candela settings may prevent the
variance in pulse widths at the different candela settings.
[0033] As discussed in more detail below, the LED flash circuit
controller 26 is configured to control the LED strobe element and
associated circuitry so that the output of the LED strobe element
has a fixed pulse width for multiple candela settings (such as all
of the available candela settings for strobe device 30). In one
embodiment, the LED flash circuit controller 26 is configured to
vary the drive current to the LED to achieve the desired light
output to meet the candela setting while maintaining the fixed
pulse width (e.g., so that the variance between pulse widths for
different candela settings is less than 10 milliseconds). In one
aspect, the current may be adjusted by switching in different
resistor values (see FIG. 8), or by using an active current
regulator circuit (see FIG. 9). An active current regulator may be
adjusted by varying the reference voltage determined by resistor
dividers, DAC, PWM signal, 555 Timer output or other means.
[0034] FIG. 3 is a schematic diagram of the system of FIG. 1,
further illustrating details of a system controller and another
example of a strobe device. FIG. 3 is similar to FIG. 2 except for
the addition of pulse width selector 48. In one embodiment, the
pulse width is fixed to a single setting (as illustrated in FIG.
2). Alternatively, the pulse width may be set using pulse width
selector 48, which is configured to receive a manual indication of
the fixed width. Similar to candela selector 46, pulse width
selector 48 may comprise one or more switches, such as a set of
jumpers or a DIP switch, thereby allowing selection of one of
multiple options for the fixed pulse width (such 50 milliseconds,
60 milliseconds, 70 milliseconds, etc.). LED flash circuit
controller 26 is configured to determine the fixed pulse width
setting (such as via the pulse width selector 48 or via a pulse
width selection command from an alarm control panel (such as system
controller 14), with the pulse width selection command indicative
of the fixed width). The LED flash circuit controller 26 is further
configured to control the LED in order to generate an output at the
desired candela setting and with a pulse width of fixed duration as
set by the fixed pulse width setting.
[0035] FIG. 4 illustrates an expanded block diagram of the strobe
device illustrated in FIG. 2. The network interface 24 includes a
strobe power control input 60 that receives the command to activate
the strobe device 30 and receives power to power the strobe device
30. The strobe power control input 60 sends the received command to
the LED flash circuit controller 26. The LED flash circuit
controller 26 includes LED control drive 58 and flash timing
control 62, which controls the timing of the flashes of the LED
strobe element. The flash timing control 62 may receive as an input
the candela selector 46, which may be an input device on the strobe
device 30 (such as a multi-position switch). An example of the
multi-position switch is disclosed in U.S. Pat. No. 7,456,585,
incorporated by reference herein in its entirety. Examples of
candela settings include 15, 30, 75, and 110. Based on the candela
setting, the flash timing control 62 may control the strobe element
and associated circuitry 44 to generate an output with the desired
candela setting. One example of the strobe element and associated
circuitry 44 is illustrated in FIG. 4, which includes an LED flash
circuit 56, a power conversion circuit 52, and energy storage
circuit 54. The power conversion circuit 52 provides the proper
regulated voltage to the energy storage circuit 54. An example of
the power conversion circuit 52 may be a voltage regulator (such as
a DC-DC converter or current regulator), and an example of the
energy storage circuit 54 may be a capacitor. The flash timing
control circuit 62 generates an output to the LED control drive 58.
Based on the output, the LED control drive 58 provides the proper
current to the LED flash circuit 56 in order for the LED flash
circuit 56 to generate the desired intensity. Further, the flash
timing control 62 generates an output to LED flash circuit 56,
which dictates the duration of the output of the LED flash circuit
56. Thus, the flash timing control 62 may control both the
intensity and the duration in order generate an output with the
requested candela rating (as dictated by candela selector 50) and
at the fixed pulse width. The flash timing control 62 further may
communicate with the power conversion circuit 52 in order for the
power conversion circuit 52 to provide the proper voltage to energy
storage circuit 54.
[0036] Thus, upon receiving the activation signal (such as in the
form of a command received by network interface 24), the power
conversion circuit 52 may charge up the storage capacitor in energy
storage circuit 54. Alternatively, the power conversion circuit 54
may charge up the storage capacitor in energy storage circuit 54
prior to receipt of the activation signal. Regardless, the strobe
element may be activated in response to receipt of the activation
signal. When the strobe element is activated, the flash timing
control 62 may initialize the power conversion circuit 52 to charge
the energy storage circuit 54, as well as configure the LED control
drive 58. This may be applicable to a notification appliance that
is addressable. In a non-addressable notification appliance, the
flash timing control may be set directly (such as locally on the
non-addressable notification appliance).
[0037] FIG. 5 illustrates an expanded block diagram of the strobe
device illustrated in FIG. 3. FIG. 5 is similar to FIG. 4 except
for the addition of pulse width selector 48.
[0038] As discussed above, the LED flash circuit controller 26 may
generate a LED output that, to the human eye, is of a fixed pulse
duration. In one embodiment, the LED flash circuit controller 26
generates a continuous output for the entire fixed pulse duration,
which is illustrated in FIG. 6a-b. The LED generates an output for
X mSec in a first candela setting (as shown in FIG. 6a) and in a
second candela setting (as shown in FIG. 6b). The intensity output
is "Y" for FIG. 6a and "Z" for FIG. 6b, with Y>Z, so that the
candela output for FIG. 6a is higher than the candela output for
FIG. 6b. The output of the LED is repeated every 1 second, as
illustrated in FIGS. 6a-b. As shown in FIGS. 6a-b, the pulse width
for the different candela settings is the same at X mSec.
[0039] In another embodiment, the LED flash circuit controller 26
generates a flickering output for the fixed pulse duration, which
is illustrated in FIG. 7a-b. The LED generates an output for X mSec
in a first candela setting (as shown in FIG. 7a) and in a second
candela setting (as shown in FIG. 7b). The intensity output is "Y"
for both FIG. 7a and FIG. 7b; however, the LED output for FIG. 7b
is pulsed, so that the candela output for FIG. 7a is higher than
the candela output for FIG. 7b. For example, the LED is controlled
so that the output is not continuous (such as being switched
on/off). The control of the LED is at a fast enough rate so that
the human eye cannot register the flicker. Instead, the human eye
perceives the average of the light output from the LED. Typically,
a rate above 60 Hz is a sufficient switching rate so that the eye
cannot register the flicker. The output of the LED is repeated
every 1 second, as illustrated in FIGS. 7a-b. Further, the pulse
width for the different candela settings is the same at X mSec. In
this way, FIG. 7b illustrates a pulse width modulation. Further,
the duty cycle for the pulse width modulation describes the
proportion of the "on" time to the "off" time and dictates the
candela output of the LED.
[0040] As discussed above, there are multiple ways in which to
control the LED in order to generate the desired LED output for the
fixed pulse width duration. FIG. 8 is one example, illustrating a
block diagram of the energy storage, LED, and flash control
circuitry in which the current is set via resistor selection.
Energy storage 802 may supply energy to LED 804. Resistors R1, R2,
R3, Rn may be sized to allow for the desired drive current
selection. FIG. 8 depicts 4 resistors; however, fewer or greater
number of resistors may be used. Switches S1, S2, S3, S4 may be
controlled by an LED drive current selection. As shown in FIG. 8,
switch S1 is closed while the remaining switches are open. Switches
S1, S2, S3, S4 may comprise a FET, driver array or the like. In
operation, the switches are configured to control the amount of
current drawn through LED 804 and for the fixed pulse duration in
order for the LED to generate the desired output at the fixed pulse
duration. For example, all selected switches are on (i.e., closed)
for the fixed pulse duration (e.g., 50 mSec), after which, all of
the switches are opened.
[0041] In one embodiment, resistors R1, R2, R3, Rn are the same
values. So that, turning on more resistors results in drawing more
current, and turning on fewer resistors results in drawing less
current. For example, if a small light output is desired, only one
switch is closed. If it is desired to double the light output (and
the resistors are the same value), two switches are closed. In an
alternate embodiment, resistors of different values may be
used.
[0042] FIG. 9 is another example of a block diagram of the energy
storage, LED, and flash control circuitry in which the current is
selected via active current regulator. In particular, instead of
having multiple resistors and multiple switches, an active current
regulator 902 may be used. The active current regulator 902 may be
configured to maintain the desired current to draw so that the LED
generates the desired output. In particular, the active current
regulator performs current threshold control, setting the limit of
the current regulator to create the desired light output from the
LED. The threshold may be the setting of a resistor divider or
ladder, or the output of a digital to analog converter (DAC), or a
pulse width modulator (PWM). Further, switch 904 may be used for
flash control, in which the switch is closed for the fixed pulse
width (e.g., 50 mSec). In this way, switch 904 is closed for the
same length of time for all candela settings of strobe device.
[0043] While the invention has been described with reference to
various embodiments, it should be understood that many changes and
modifications can be made without departing from the scope of the
invention. It is therefore intended that the foregoing detailed
description be regarded as illustrative rather than limiting, and
that it be understood that it is the following claims, including
all equivalents, that are intended to define the spirit and scope
of this invention.
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